Hydraulic fracturing is a common technique used to improve production from existing wells, low rate wells, new wells and wells that are no longer producing. Fracturing fluids and fracture propping materials are mixed in specialized equipment then pumped through the wellbore and into the subterranean formation containing the hydrocarbon materials to be produced. Injection of fracturing fluids that carry the propping materials is completed at high pressures sufficient to fracture the subterranean formation. The fracturing fluid carries the propping materials into the fractures. Upon completion of the fluid and proppant injection, the pressure is reduced and the proppant holds the fractures open. The well is then flowed to remove the fracturing fluid from the fractures and formation. Upon removal of sufficient fracturing fluid, production from the well is initiated or resumed utilizing the improved flow through the created fracture system. In some cases, such as recovering natural gas from coal bed methane deposits, proppants are not applied and the simple act of fracturing the formation suffices to provide the desired improvement in production. Failure to remove sufficient fracturing fluid from the formation can block the flow of hydrocarbon and significantly reduce the effectiveness of the placed fracture.
Selection of the fracturing fluid is important and dependent upon the objectives of, and expectations from, the treatment. A properly selected fluid will be cost effective, capable of being mixed and pressurized at surface, reliably carry proppant into the fractures, minimize damage and blockage in the pores of the formation, be readily handled at surface during recovery and minimize environmental impact of the overall operation. Selection of a fracturing fluid to best meet these sometimes conflicting requirements is difficult and care must be taken. To best meet these requirements over the wide range of situations and needs, many fracturing fluids and fracturing fluid mixtures have been disclosed to complete hydraulic fracturing, predominately based on water and hydrocarbon fluids with and without addition of a gas phase such as nitrogen or carbon dioxide.
The use of nitrogen or carbon dioxide with oil or water based fracturing fluids can provide a range of benefits. Presence of these gases improves or quickens fracturing fluid removal from the formation such that better effectiveness of the fracturing treatment is achieved. Additionally, these gases reduce the density of the base fracturing fluid allowing improved flow along the wellbore, again improving or quickening fracturing fluid removal. Interactions between the nitrogen or carbon dioxide and the base fracturing fluid can alter the physical properties of the base fluid including reducing the mixture viscosity, minimizing relative permeability effects and lowering surface tension. These effects result in improved fracturing fluid mobility within the formation pores, improved recovery of the fracturing fluids and superior production. In the case of hydrocarbon based fluids, the nitrogen or carbon dioxide may be less expensive and provide a cost effective volume substitution thereby lowering the cost of the treatment. Water, though often inexpensive to acquire, may be subject to significant costs for disposal of the recovered fluid and can have a negative environmental impact associated with their consumption, handling, recycling or disposal once recovered. Again nitrogen or carbon dioxide can provide benefit by reducing the volume of fluid applied, recovered and needing disposal.
However, the use of nitrogen or carbon dioxide based fracturing fluids can have detrimental effects on the hydraulic fracturing process. During fracture fluid recovery, nitrogen or carbon dioxide contaminate the natural gases produced, and without processing to remove the contamination, need to be flared or vented until the concentration of the fracturing gases in the well production fluid stream is sufficiently reduced that the produced well stream can be commercialized. Flaring and venting of the natural gas component during this period wastes this stream while is also environmentally damaging.
Carbon dioxide is chemically active in water and hydrocarbon based fluids. Solution of carbon dioxide in water forms carbonic acid requiring alterations to the viscosity developing chemicals often added. Further, the increased water acidity can interact with minerals in the formation resulting in damage and reduced effectiveness of the fracturing treatment. Upon recovery, the formed carbonic acid also promotes corrosion in surface equipment and pipelines. In hydrocarbon based fluids the addition of carbon dioxide impairs the effectiveness of the viscosity developing chemicals and may result in poor performance of fluid for creating the fractures and carrying the proppant into the fractures. Nitrogen is much more inert than carbon dioxide in water and hydrocarbon based liquids, however is gasified prior to injection and often results in reduced fluid hydrostatic pressures, increased injection pressure at surface and additional equipment or altered wellbore design to accept the higher injection pressures. Further, the source of the nitrogen or carbon dioxide is often remote from the well and suitable supply of these gases can make their application too expensive for good well economics. This is particularly true where it is necessary to economically generate multiple large fracture networks that are needed to effectively stimulate less prolific reservoirs such as tight gas and shale gas. With these challenges, the industry needs better solutions to maximize production from oil and gas formations, reduce or eliminate water use and eliminate venting and flaring. The solutions must be cost effective and minimally disturbing to allow quick and effective implementation and adoption.